Trigger documentation? Need more examples.. pleeeze.. ;-)

Hello PostgreSQL friends!

I'm new to PostgreSQL, but I've been using MySQL/mSQL for a long time now. I
switched to PostgreSQL mostly because of it's features: triggers, specially.

I've studied, compiled and tried the examples from the crontrib/spi directory.
They all worked, but without some kind of documentation it is really hard to
understand how things really work, and how I can create my own stuff.

In the "Programmer's Guide", there's a whole chapter documenting the SPI
library, but I didn't find any documentation of the functions directly related
to TRIGGERS (library "triggers.h"), like: "TRIGGER_FIRED_BY_UPDATE()",
"CurrentTriggerData->tg_newtuple", etc...

Is this documentation available somewhere?

-----

I have this table:

CREATE TABLE users (
userid INT4 NOT NULL,
fullname CHAR(40),
username CHAR(20),
password CHAR(13)
)

Using the examples from contrib/spi, I guess I can manage to create an INSERT
and DELETE trigger. But I'm not sure how to create an UPDATE one.

Let's suppose someone updates both fields "username" and "password". HOW from
the trigger I know which fields changed. Is it possible to get the older
and newer values and compare them?

This UPDATE trigger doesn't need to modify it's own new inserted data, but it
has to modify external data (create/delete files at my Linux filesystem,
rename directories, etc..).

Is such thing possible? Someone has an example of this? ;-)

Thanks a lot!

Regards from Brazil!

--
Henrique Pantarotto
CEPAnet Internet Provider
webmaster / analista de sistemas
Email: scanner(at)cepa(dot)com(dot)br
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